Photolithography provides tools and techniques for patterning the various layers that make up semiconductor integrated circuits. In general, a mask layer of photosensitive material is deposited onto the semiconductor wafer that is to be patterned. Then, the photolithographic tool is used to expose predetermined areas of the photosensitive mask material thereby “printing” a mask pattern onto the photosensitive layer. After exposure, the photosensitive layer is “developed” and the unexposed regions are removed thereby leaving openings through which further processing of the semiconductor wafer can take place. The pattern on the wafer can be generated in the photo-resist layer by projecting the pattern onto the material through a mask having the appropriate design.
Masks are also patterned with photo-resist by printing them with a process mask like a laser printer prints a pattern on paper. That is, the pattern generator tool simply scans the surface of the photo-resist with a light beam having a wavelength to which the photo-resist is sensitive. Depending on the characteristics of the photo-resist, different sources of light or radiant energy are used, including ultraviolet light, visible light, coherent light, x-rays and electron beam (E-Beam).
High throughput laser pattern generators have been developed to print onto the photo-resist with multiple beams so as to increase their area coverage rate (i.e., throughput). In such systems, an array of beams is generated by using one of several different techniques. One technique involves using assemblies of discrete mirrors and beam splitters. Another involves using an assembly of plates of different thicknesses to produce beams separated a controlled distance by using front and back surface reflections. (See U.S. Pat. No. 4,797,696). Still another technique involves using a diffractive optical element to generate a fan of beams that is focused to an array using a lens. The individual beams then pass through a multiple channel acousto-optic modulator to turn the beams on and off or to impart gray levels to them. The array is scanned down stream in the writing system by some deflection means, typically a rotating mirror polygonal scanner or an acousto-optic deflector. Such systems are described in U.S. patent application entitled Laser Pattern Generator, Skjerven Morrill Docket M-5487, and U.S. Pat. Nos. 4,796,038; 5,386,221; and 5,635,976.
The accurate placement of the beams on the work piece is essential for writing patterns for producing semiconductors. Errors in stage position or beam wander can be compensated in the scan direction by adjusting the timing of the modulation of the beams. A piezo-electric actuated mirror is typically used in the horizontal axis or cross-scan but these are limited in frequency response to about 1 kHz.
The control of laser beam intensity noise is important so that all features receive the desired dose and have the same dimension when the resist is developed.